I've got a strong private sense of what a subplot is, what one's purpose is, how to go about one. My primary interest is gathering other perspectives on what subplot is. I've surely run into many a gamut of parts and parcels of what plot means. Although I know plot as a structural shape, the aesthetics of complication, character, setting, event, and idea, at least, contribute to a broader picture of what plot means.

To me, a subplot can be many things. Perhaps it simply resembles a very small plot of its own--a 'sideplot,' if you will. A minor issue, only related to the plot in that it involves one or more of the main characters. Perhaps it is a minor theme that parallels the plot, for example a task the MC and co. must accomplish on their way to accomplishing their greater mission. Perhaps it isn't minor at all, but is only slightly less lengthy or important than the main plot. Sometimes these can be completely separate from the main plot, and they may conflict one another to build tension.

But as with all subplots, it must truly be "sub"--that is, below the main plot in the pecking order. Perhaps it is an issue that is more important to the characters, but the issue of the main plot must be resolved first, hence giving the main plot higher priority despite their reluctance. This is the kind I find myself using in my current WIP. There's a terrible accident, and a character (directly related to both the MC and the antihero) is...lost, for the sake of brevity (it's a rather complicated matter). Both the MC and the antihero go about searching for her, but in both cases there are more pressing demands to be met before they can be freed to complete their search. This is something they struggle with throughout the book, often leading to arguments of varying volatility, so it underscores their determination to finish the current plot so they can go find her.

These are surely but a few of the many species of subplot out there. Who else can give us insight to this incredible creature?

When I hear subplot, I think of RPGs. It's those sidequests you have to do that deviate you from what you thought was the main goal, but that brings you to the ultimate purpose.

Say for instance, you have Main Hero\Heroine. They think their main goal is to get to the castle to save the king. But OMG little girl is crying and needs her berries. The bad monster stole them!

So you go to slay bad monster. This is a side quest\subplot. But from the monster you learn he's one of many that will come to rise the BIG DRAGON from the dead and DESTROY THE WORLD.

Your ultimate goal has been found. You must SAVE THE WORLD.

After side questing to save the king, of course. XD

That's a bad description, but it gives you an idea. Books are (hopefully) usually written better than that, with the goals and sidequest\plots woven together so intricately that you're turning the page, wanting to know what happens next to everyone.

I tend to think of them as secondary stories related to the main plot and used to express variations on the main theme that the main plot is too busy to explore. I have them do double duty by using them for additional development of secondary characters.

Rather than "bad," I think I would call that example "So Bad It's Good." I laughed when I read it, because I've come across such things countless times. Pulled off well, they're actually somewhat refreshing, maybe because they give the reader the idea that "hey, big things can really come from small deeds!" Rather than the typical idea of "you have to be a hero to do this kind of stuff." It takes away the You Must Be This Tall To Advance The Plot sign. :)

Rather than "bad," I think I would call that example "So Bad It's Good." I laughed when I read it, because I've come across such things countless times. Pulled off well, they're actually somewhat refreshing, maybe because they give the reader the idea that "hey, big things can really come from small deeds!" Rather than the typical idea of "you have to be a hero to do this kind of stuff." It takes away the You Must Be This Tall To Advance The Plot sign. :)

LOL. Thank you! I have to ask though, is that a Trope? I feel myself being lured to the website...

What a trope is might be another topic, though related in a sense to subplot. A literary trope, for example, is a recurring thematic feature of plots and subplots. For instance, boy meets girl, girl falls in love with boy, boy loves another girl or boy, and so on to a final outcome. Motif and trope in some senses have similar connotations. Spaceships are a motif, their fantastical fiction capabilities like faster than light speed travel are the stuff of science fiction tropes.

Stock complications and stock high-concept premises are cores of convention-driven literature, and tropes for being recurring motifs.

Stereotypes, stock characters, archetypes are recurring motifs in the sense of substitution schemes, where a standard character type substitutes for a proxy reality of a real-to-life person character's personality. Therefore, tropes.

Stock settings with emotional baggage and accepted meanings are tropes in the sense they recur throughout a literary era or across the opus of literature.

Stock events, stock ideas are tropes in the sense they recur as well.

Stock circusmstances transcend routine everyday realities and become participation mystiques from unique perspectives of the ordinary. Arguably, that's a trope in and of itself.

So trope isn't off topic in my opinion. At least as concerns complications, characters, settings, events, and ideas' influences on plots and subplots.

Yes, TVTropes has several connotative spins on what a trope is, but they're limiting, in my opinion. The denotative meaning of trope and its many species as opposed to and part of rhetorical scheme and figures of speech, for example, is largely overlooked there.

I guess I view subplots sort of like tributaries of a stream. They are separate smaller stories running alongside the main plot. But certainly by the end of the novel I expect them to meet up in some way. I try to structure mine so that they round out the characters and fill in aspects of the world I've created that would otherwise be ignored by the main plot.

I'm curious to know what people's opinions are on how many subplots are enough? Can there be too many subplots? How do you figure out the balance?

I've examined a number of novels for how many subplots they have. Four concurrent subplots seems to be an upper limit, plus the main plot. I'm not counting analepsis and prolepsis subplots; in other words, flashback and flash forward and other anachronies.

I have trouble with that same question too. My WIP is (intended to be) the first in a series, so I've had a greater chance to really sit and be able to develop whatever subplots may eventually come into play (typically character-driven ones, as the overarching plot gets foggier as it goes deeper into as-yet uncharted territory)*. The trouble begins when I have to decide which developing subplots to put where, should they be established through dialogue or backstory techniques such as flashbacks, etc. The 'magic number' of subplots per book can also grow or shrink depending on the sizes of said subplots and their significance to the rest of the story.

So I guess the best way to reach a 'balance' is by looking at the subplots vs. the plot and selecting those that are more appropriate, such as ones which aid in character development or complement the plot in some other way. When weaving subplots into the frame, I give them exclusivity: only one sub-plot can be active at a time, with a few exceptions (one in particular, which runs under everything and might even be the main plot were it not for...well, the main plot). As far as the overall book goes, I usually limit it to around five--I think if I can keep track of them on one hand, I'm still all right. :)

But again, this is just my opinion. I'm sure it's possible to have many more than five subplots and still create a masterpiece, and if that's the way you want to do it, go ahead. It's your book; make it soar in your own fashion.